Solid solutions involving MGO-V2 O3 -TiO2

ABSTRACT

A novel oxidation catalyst or refractory and the method for its preparation. The material comprises substitutional solid solutions involving MgO, V 2  O 3  and TiO 2  and having compositions that fall within a specific polygon in a ternary composition diagram of MgO, V 2  O 3  and TiO 2 . A typical material has the composition in Wt %; MgO 17, V 2  O 3  61 and TiO 2  22, and consists of the phases spinel having the formula Mg 1 .21 V 1 .58 Ti 0 .21 O 4  and pseudobrookite having the formula Mg 0 .20 V 1 .60 Ti 1 .20 O 5 .

BACKGROUND OF THE INVENTION FIELD OF THE INVENTION

This invention relates to substitutional solid solutions involving MgO-V₂ O₃ -TiO₂.

SUMMARY OF THE INVENTION

The present invention pertains to a novel oxidation catalyst and the method for its preparation. The material comprises substitutional solid solutions involving MgO, V₂ O₃ and TiO₂ and having compositions that fall within a polygon in a ternary composition diagram of MgO, V₂ O₃ and TiO₂ as described in the drawing herein; wherein the polygon has the vertices and corresponding coordinates in weight percent as shown in Table I.

    ______________________________________                                                 Coordinates (wt %)                                                     Vertices  MgO           V.sub.2 O.sub.3                                                                        TiO.sub.2                                      ______________________________________                                         A         30            70                                                     B         42            28      30                                             C         42            16      42                                             D         14            16      70                                             F          3            27      70                                             G          3            97                                                     ______________________________________                                    

The substitutional solid solutions involving MgO, V₂ O₃ and TiO₂ having compositions falling within the polygon shaped shaded area in the ternary composition diagram of MgO V₂ O₃ and TiO₂ as shown in the drawing are made by the process comprising the steps of:

(1) heating V₂ O₅ powder having a particle size of less than about 50 microns in an atmosphere in which the partial pressure of oxygen is in the range of about 10⁻⁸ to 10⁻¹⁰ atmosphere and the CO₂ /H₂ volume ratio is in the range of about 10/1 to 1/1 while gradually increasing the temperature stepwise over a temperature range of about 600° C. to 1300° C. and holding the final temperature for a period of at least about 24 hrs. to ensure complete reduction of all vanadium to V³⁺ ;

(2) heating TiO₂ and MgO powders each having a particle size of less than about 50 microns for a period of about 10 to 14 hrs. at a temperature in the range of about 600° C. to 800° C. for TiO₂ and about 1000° C. to 1200° C. for MgO;

(3) thoroughly grinding together about 97 TO 16 wt.% of V₂ O₃ from (1), with about 0 TO 70 wt.% of TiO₂ and about 3 to 42 wt.% of MgO from (2) to produce a mixture having a grain size of less than about 50 microns,

(4) pelletizing the mixture from (3) at a pressure of about 5,000 psi; and

(5) heating the pellets from (4) at a temperature in the range of about 1100° C. to 500° C. for a period in the range of about 12 to 48 hrs. in an atmosphere in which the partial pressure of oxygen is in the range of 10⁻¹⁰ to 10⁻⁵ atmosphere to produce said solid solutions involving MgO, V₂ O₃ and TiO₂.

In one embodiment, the pellets from step (5) are cooled to ambient temperature while at substantially the same partial pressure of oxygen as in step (5). Preferably, to prevent undue oxidation and moisture pickup, the cooled pellets are stored in a sealed container.

BRIEF DESCRIPTION OF THE DRAWING

The drawing depicts a ternary composition diagram showing a polygon within which useful substitutional solid solutions involving MgO-V₂ O₃ -TiO₂ are found.

DESCRIPTION OF THE INVENTION

New solid solutions have been synthesized by the subject invention in which the nature and extent of the formation of substitutional solid solutions involving vanadium oxide, notably V₂ O₃, as component have been characterized. This work has led to synthesizing new phases in which vanadium in several oxidation states are accommodated such as to render crystalline phases thermodynamically stable over a wide range of oxygen pressures at elevated temperatures.

The newly synthesized substitutional solid solutions in the MgO-V₂ O₃ -TiO₂ system are shown as falling within the polygon shaped shaded area in the ternary composition diagram in the drawing. The oxidation states of vanadium in the substitutional solid solutions are mainly +3 and +4. The polygon has the following vertices and corresponding coordinates in weight percent:

                  TABLE II                                                         ______________________________________                                                 Coordinates (wt %)                                                     Vertices  MgO           V.sub.2 O.sub.3                                                                        TiO.sub.2                                      ______________________________________                                         A         30            70                                                     B         42            28      30                                             C         42            16      42                                             D         14            16      70                                             F          3            27      70                                             G          3            97                                                     ______________________________________                                    

A typical newly synthesized material consisting of two solid solutions, viz, a spinel having the formula Mg₁.21 V₁.58 Ti₀.21 O₄ and pseudobrookite having the formula Mg₀.20 V₁.60 Ti₁.20 O₅ is illustrated by point H in the drawing having the following coordinates in wt.%: MgO 17, V₂ O₃ 61, and TiO₂ 22.

Substitutional solid solutions in the MgO-V₂ O₃ -TiO₂ system are synthesized in the following manner: First, V₂ O₃ is prepared by heating commercially available analyticalgrade V₂ O₅ having a particle size of less than about 50 microns in a vertical tube furnace in an atmosphere of carefully selected oxygen pressure within the stability range of V₂ O₃ e.g. in the range of 10⁻⁸ to 10⁻¹⁰ atmospheres. This is accomplished by using a gas mixture of high-purity CO₂ and H₂ in controlled proportions. The CO₂ /H₂ ratio by volume is in the range of 10/1 to 1/1. The relatively low melting point of the starting vanadium oxide (V₂ O₅), e.g. about 690° C., necessitates heating the oxide slowly. Starting at a temperature of about 600° C., the temperature is gradually increased stepwise over a period of about 12 to 24 hrs. to a final temperature of about 1300° C. At that temperature the oxide is held at least about 24 hrs. e.g. 24 to 30 hrs. to ensure complete reduction of all vanadium to V³⁺ (V₂ O₃). The identity of the reaction product may be confirmed by X-ray diffraction analysis.

At subsolidus temperatures, this system is dominated by the spinel MgV₂ O₄ -Mg₂ TiO₄) and pseudobrookite (V₂ TiO₅ -MgTi₂ O₅) solid solutions. The former solid solution (spinel) is complete between the two end members at high temperatures (>1400° C.) whereas the latter solid solution (pseudobrookite) extends for most of the composition range from MgTi₂ O₅ toward V₂ TiO₅, but connects to a different phase assemblage(s) near the V₂ O₃ -TiO₂ point.

The rhombohedral solid-solution phases of corundum-type structure extend relatively short distances into the "ternary" system from their respective end members V₂ O₃ and MgTiO₃. In the central part of the "ternary system", the solid solution between these two end members is unstable relative to the phase assemblage spinel solid solution plus pseudobrookite solid solution and hence decomposes to these two phases.

The distribution of vanadium among the phases spinel, pseudobrookite and sesguioxide (=rhombohedral phase) will be relatively sensitive to variations in oxygen pressure. Hence, this distribution will give important information on crystal-chemical and the energetic' aspects of the substitution of vanadium in its various oxidation states, especially in the spinel phase.

Pre-heated analytical-grade oxides having a particle size of less than about 50 microns are used as starting materials for the other components of the solid solutions to be synthesized. For example, TiO₂ is heated for about 10 to 14 hrs. at a temperature in the range of about 600° C. to 800° C. and MgO is heated for about 10 to 14 hrs. at a temperature in the range of about 1000° C. to 1200° C. prior to being used in preparation of the final mixtures.

The mixtures of the three oxide components are mechanically ground together under acetone in an agate mortar to ensure thorough mixing and a sufficiently small grain size e.g. less than 50 microns. For example, about 97 to 61 wt.% of V₂ O₃ is ground together with about 0 to 70 wt.% of TiO₂ and about 3 to 42 wt.% of MgO to produce a mixture having a grain size of less than about 50 microns. Complete formation of the desired compounds in the succeeding heat treatment is thereby promoted. Next, the oxide mixtures are pelletized at a pressure of about 5,000 psi or higher. The pellets may have any conventional size e.g. 1/16" to 1". The pellets are then heated at a temperature in the range of about 1100° C. to 1500° C. for a period in the range of about 12 to 48 hrs. (depending on the nature and solidus temperature of the phase to be synthesized) in a vertical tube furnace with carefully controlled oxygen pressures, in the range of 10⁻¹⁰ to 10⁻⁵ atmosphere. In the manner described previously for preparing V₂ O₃, the pellets are heated in a furnace atmosphere provided by a gas mixture of CO₂ and H₂ in various desired mixing ratios, typically in the range of about 10/1 to 1/1. These selected ratios are kept constant for the duration of the synthesis by use of a differential manometer. By this method the oxygen pressure at the specified temperature can be controlled to better than ±1%. The thermodynamic data for the water-gas reaction (CO₂ +H₂ =CO+H₂ O), on which the calculations were based, are known with extremely high accuracy (better than 0.1%), and hence the method used ensures reliable accurate control of the oxidation state of vanadium during the synthesis. This is extremely important for optimization of the properties of the finished product.

In addition to oxygen pressure, one other parameter influencing the oxidation state of vanadium in oxide phases has been utilized in synthesizing the new phases, viz. the provision of host structures which incorporate vanadium in one (or more) valence state(s) in strong preference to other valence states. In the present case, the extensive ternary solid solution phases of pseudobrookite type and of spinel type structures in the system MgO-V₂ O₃ -TiO₂ serve this purpose. In these solid solutions the vanadium is present partly in the trivalent state, as stabilized by the preferential substitution of V³⁺ for Al³⁺, and partly in the tetravalent state, as stabilized by the preferential substitution of V⁴⁺ for Ti⁴⁺. Furthermore, MgO in combination with vanadium oxide stabilizes V⁵⁺ in the crystalline phase Mg₃ V₂ O₁₂ by virtue of the fact that MgO is a base relative to V₂ O₅.

The pellets of the substitutional solid solutions involving MgO-V₂ O₃ -TiO₂ may be used as an oxidation catalyst, offering improved activity and yields and greater stability over a wider temperature range e.g. about 1000° C. or higher than that which is offered by typical oxidation catalysts. For example, as an oxidation catalyst the subject pellets may be used in the conversion of o-xylene to phthalic anhydride, butane to maleic anhydride, or alcohols to aldehydes or organic acids.

As a high temperature refractory or as an ingredient in a high temperature refractory material for lining a gasifier for the partial oxidation of ash-containing liquid hydrocarbonaceous and solid carbonaceous fuels, the subject material would have a long life, withstanding attack from V₂ O₅. For example, a thermal refractory for lining the reaction zone of a partial oxidation gas generator may comprise the previously described solid solutions involving MgO-V₂ O₃ -TiO₂ in the amount of about 1 to 25 wt.%, and the remainder comprising at least one other oxide of one of the elements Cr, Mg, Al, Mn, Si, or mixtures thereof.

The subject solid solutions involving MgO-V₂ O₃ -TiO₂ may also be used as an additive. About 0.01 to 0.2 wt.% (basis weight of ash in the fuel feed) of the additive may be introduced into the refractory-lined partial oxidation gas generator along with the fuel feed to stabilize said refractory.

Various modifications of the invention as herein before set forth may be made without departing from the spirit and scope thereof, and therefore, only such limitations should be made as are indicated in the appended claims. 

We claim:
 1. Compositions produced from MgO, V₂ O₃ and TiO₂ wherein said compositions fall within the shaded area circumscribed by ABCDFG as shown in the disclosed Figure wherein the amounts of MgO, V₂ O₃ and TiO₂ are characterized as shown in the table below:

    ______________________________________                                                 Coordinates (Wt %)                                                     Vertices  MgO           V.sub.2 O.sub.3                                                                        TiO.sub.2                                      ______________________________________                                         A         30            70                                                     B         42            28      30                                             C         42            16      42                                             D         14            16      70                                             F          3            27      
 70.                                           G          3            97                                                     ______________________________________                                    


2. A material consisting of two compositions as provided in claim 1 consisting of a spinel having the formula Mg₁.21 V₁.58 Ti₀.21 O₄ and pseudobrookite having the formula Mg₀.20 V₁.60 Ti₁.20 O₅ and the gross composition represented by point H in said disclosed figure and having the following coordinates in wt.%: MgO 17, V₂ O₃ 61 and TiO₂
 22. 3. A composition as provided in claim 1 wherein said vanadium is present in the trivalent an tetravalent states.
 4. A composition as provided in claim 1 having a structure comprising pseudobrookite.
 5. A method of preparing compositions from MgO, V₂ O₃ and TiO₂ that fall within a polygon shaped area in a ternary composition diagram of MgO, V₂ O₃ and TiO₂ comprising the steps of(1) heating V₂ O₅ powder having a particle size of less than about 50 microns in an atmosphere in which the partial pressure of oxygen is in the range of 10⁻⁸ to 10⁻¹⁰ atmospheres while gradually increasing the temperature stepwise over a temperature range of about 600° C. to I300° C. and holding the temperature at about 1300° C. for a period of at least about 24 hrs. to ensure complete reduction of all vanadium to V³⁺ ; (2) heating TiO₂ and MgO powders each having a particle size of less than about 50 microns for a period of about 10 to 14 hrs. at a temperature in the range of about 600° C. to 800° C. for TiO₂ and about 1000° C. to 1200° C. for MgO; (3) thoroughly grinding together about 97 to 16 wt.% of V₂ O₃ from (1), with about 0 to 70 wt.% of TiO₂ and about 3 to 42 wt.% of MgO from (2) to produce a mixture having a grain size of less than about 50 microns, (4) pelletizing the mixture from (3) at a pressure of about 5,000 psi; and (5) heating the pellets from (4) at a temperature in the range of about 1100° C. to 1500° C. for a period in the range of about 12 to 48 hrs. in an atmosphere in which the partial pressure of oxygen is in the range of 10⁻¹⁰ to 10⁻⁵ atmosphere.
 6. The method of claim 5 further comprising cooling the pellets from (5) to ambient temperature, while at substantially the same partial pressure of oxygen as in (5); and storing the cooled pellets in a sealed container.
 7. The method of claim 5 where in step (1) said V₂ O₅ powder is heated in an atmosphere comprising a gas mixture of CO₂ and H₂ with a volume ratio of CO₂ /H₂ in the range of about 10/1 to 1/1.
 8. The method of claim 5 where in step (5) said pellets are heated in an atmosphere comprising a gas mixture of CO₂ and H₂ with a volume ratio of CO₂ /H₂ in the range of about 10/1 to 1/1.
 9. The method of claim 11 wherein the compositions produced from MgO, V₂ O₃ and TiO₂ fall within the shaded area circumscribed by ABCDFG as shown in the disclosed Figure wherein the amounts of MgO, V₂ O₃ and TiO₂ are characterized as shown in the table below:

    ______________________________________                                                 Coordinates (Wt %)                                                     Vertices  MgO           V.sub.2 O.sub.3                                                                        TiO.sub.2                                      ______________________________________                                         A         30            70                                                     B         42            28      30                                             C         42            16      42                                             D         14            16      70                                             F          3            27      
 70.                                           G          3            97                                                     ______________________________________                                    


10. The method of claim 9 wherein said compositions have the formulas Mg₁.21 V₁.58 Ti₀.21 O₄ (spinel) and Mg₀.20 V₁.60 Ti₁.20 O₅ (pseudobrookite) and the composition represented by point H in the disclosed Figure and having the following coordinates in wt.%: MgO 17; V₂ O₃ 61, and TiO₂
 22. 11. The method of claim 11 wherein said compositions produced from MgO, V₂ O₃ and TiO₂ have structures of pseudobrookite or spinel.
 12. An oxidation catalyst produced from MgO, V₂ O₃ and TiO₂ wherein the composition of said oxidation catalyst falls within the shaded area circumscribed by ABCDFG as shown in the disclosed Figure wherein the amounts of MgO, V₂ O₃, and TiO₂ are characterized as shown in the table shown:

    ______________________________________                                                 Coordinates (Wt %)                                                     Vertices  MgO           V.sub.2 O.sub.3                                                                        TiO.sub.2                                      ______________________________________                                         A         30            70                                                     B         42            28      30                                             C         42            16      42                                             D         14            16      70                                             F          3            27      
 70.                                           G          3            97                                                     ______________________________________                                    


13. Thermal refractory compositions comprising, ( 1) a solid solution in the amount of about 1-25 wt%, and (2) at least one oxide having a cationic portion selected from the group consisting of Cr, Mg, Al, Mn, Si, and mixtures thereof, wherein said solid solution is produced from MgO, V₂ O₃ and TiO₂ and has a composition that falls within the shaded area circumscribed by ABCDFG as shown in the disclosed Figure wherein the amounts of MgO, V₂ O₃ and TiO₂ are characterized as shown in the table below:

    ______________________________________                                                 Coordinates (Wt %)                                                     Vertices  MgO           V.sub.2 O.sub.3                                                                        TiO.sub.2                                      ______________________________________                                         A         30            70                                                     B         42            28      30                                             C         42            16      42                                             D         14            16      70                                             F          3            27      
 70.                                           G          3            97                                                     ______________________________________                                     